Approximately 14% of troops returning from Iraq and Afghanistan suffer from posttraumatic stress disorder (PTSD); a substantial minority of these also suffer from traumatic brain injury (TBI). Even mild TBI (mTBI), the most common type of TBI, entails some degree of neurocognitive impairment. Both PTSD and TBI frequently cause significant functional impairment. Effective treatments for PTSD exist, with cognitive behavioral therapy (CBT) being supported by the most evidence. However, CBT relies at least in part on intact cognition to be effective, and it is unclear whether individuals with brain damage are able to benefit from it. This question represents an important knowledge gap in PTSD therapeutics and will be studied here. There is limited evidence that certain pretreatment characteristics make it less likely that PTSD individuals will benefit from CBT. However, there is no information regarding how to predict response to treatment in individuals with co-existing PTSD and mTBI. Impaired cognition is associated with poorer occupational and social functioning in PTSD, as well as in TBI. There is reason to believe that cognitive impairment and/or brain dysfunction, reflected in neuroimaging, may be associated with negative PTSD treatment outcomes. Correlating neurocognitive and neuroimaging pre-treatment characteristics with CBT treatment outcomes in individuals with PTSD with and without co-morbid mTBI is an additional goal of this study. More specifically, the study aims are: A) To examine potential differences between individuals with PTSD with or without mTBI in their treatment response to cognitive processing therapy (CPT); B) To identify pre-treatment factors predictive of response to CPT. To achieve these goals, the study will recruit veterans with chronic PTSD classified into two groups: those who have sustained mTBI and who complain of current cognitive difficulties (PTSD+mTBI), and those who have not sustained a TBI (the PTSD-mTBI group). Subjects will receive 12 CPT sessions. Prior to starting CPT treatment, subjects will receive baseline neurocognitive, MRI and DTI assessments. Subjects' PTSD symptoms will be assessed at baseline, at regular intervals during CPT and sertraline treatment, and 6 months after completing treatment. Data will be analyzed according to the aims articulated above.